If you receive this message in Symantec Backup Exec and the library goes offline – the following fix worked for me:

1. Right click on the tape drive – uncheck enable.
2. Right click again – select delete.
3. Right click on the library – uncheck enable
4. Right click again – select delete.
5. Restart all Backup Exec Services

To delete the drives I had to point existing jobs to a different device, so I created a temp backup to disk device.